Based on organic light-emitting diodes (OLED), new flat displays with many advantages can be realized. Part of theses advantages is wide-area deposition on different substrates, self-illuminating characteristics, allowing very thin displays, a high independence of the angle of view and the potentially high efficiency of such displays.
For realizing such displays, structuring into different emitting elements, so called pixels, is required. This applies particularly for full color displays, where those pixels have to be realized such that they emit in different colors. Structuring via shadow masking technology can no longer be applied in small pixels in mini or microdisplays.
The possibility of producing such structures depends heavily on the used materials. On the one hand, organic light-emitting diodes have been realized based on polymers, which are often disposed as solutions. In that case, it is possible to perform structuring the pixels by printing methods. For example, it could be shown that efficient pixeling is possible by ink jet printing. In this pixeling technology, the resolution limit of the pixels is at approximately 50 to 100 μm. Organic light-emitting diodes of polymers, however, also have their disadvantages, for example a lower life span compared to vapor-deposited organic light-emitting diodes.
These printing techniques are not available for light-emitting diodes based on vapor-deposited materials. Further, it is important for the quality of the produced LEDs, to perform all structuring in vacuum.
U.S. Pat. No. 5,701,055 describes a possibility for obtaining structuring with integrated disruption edges, called separators below, for monochrome passive matrix displays, i.e. displays without individual control circuit. Here, both an organic layer and a cap electrode are structured by an undercut edge between individual rows.
However, for color displays it is necessary to realize selectively different colors. Conventionally, this is obtained by a lateral separation of a pixel into three subpixels for red, green and blue. In order to obtain sufficient luminosity of the pixel, every subpixel has to illuminate with threefold luminosity according to the area proportion of maximum one third of the total pixel, to represent a corresponding light-emitting diode of the overall pixel for the viewer. In the real case, the area proportion of a subpixel is at fewer than 33%. Due to the increased control, a higher current intensity is required for every pixel. Thereby, at the same time, the life span of the pixel is reduced.
A standard approach for structuring the vapor-deposited organic light-emitting diodes in the subpixels is based on evaporating colorants through a shadow mask. Such vapor-deposition through a shadow mask has been basically shown in the laboratory scale. However, the same is limited to displays with relatively low dimensions, since larger masks tend to distort. Further, such shadow masks tend to clog and block with colorants. This requires that the masks be frequently cleaned, which is a significant disadvantage in an industrial production process. Displays with subpixel sizes in the demonstration scale of up to 50 μm have already been presented. However, with smaller pixel sizes, this method can no longer be applied. Already with pixel sizes for so called minidisplays with a diagonal of more than “1”, only a little area of the pixel can be used for the actual OLED, since a distance between different colors has to be maintained due to the inaccuracies of the shadow masks, in order to avoid overlapping of the subpixels.
For even higher resolutions, approaches are known, which use a realization of white emitters that are correspondingly filtered by color filters. In light-emitting diodes, which emit through the substrate, so-called substrate emitters, the filters can be applied prior to depositing light-emitting diodes on the substrate. In top emitters, the filters can be applied after depositing the light-emitting diodes. A large advantage of this method is the usage of a full-area deposition of the organic layer without having to use shadow masks. A disadvantage of this method is an efficiency loss due to the color filters and an optical and/or electrical crosstalk of neighboring pixels due to the non-structuring of the organic layers.